Process for introducing an additive into a polymer melt

ABSTRACT

A process includes introducing a base polymer into an extruder, heating to a temperature sufficient to form a polymer melt, introducing a liquid such as water including an additive with mixing and subsequent vaporization and venting of the liquid to produce an extrudate having a uniform distribution of preferably uniformly small additive particles.

BACKGROUND

The present invention relates to a method of blending an additive into abase polymer. In particular, it relates to producing a homogeneouspolymer blend with an additive, especially a salt additive useful forthe manufacture of polymer films.

Melt blending polymers and functional additives to produce blendedcompounds is a common method to deliver those additives during themanufacture of plastic articles. The compound may be a concentratedcombination (or masterbatch) of the additive and a carrier materialwhereby the final amount of the additive in the article is achieved bydilution during fabrication of the article. Additionally, the compoundmay be a fully formulated blend of additives and polymers that directlyreflects the composition of the finished article. The quality andfunctionality of the article is often impacted by the uniformity of thedispersion of the additives in the compound. If the homogeneity of thedispersion is poor, it can be expected that the intended functionalityof the additive will vary. Additionally, it is possible that undesirableside effects like poor aesthetics may result from poor dispersion ofadditives.

A pellet is a convenient form for providing compounded materials. Thetraditional way to form a compounded pellet is to combine the individualcomponent materials in a high intensity mixing device. The polymericportion of the combination is melted to form a viscous liquid or “melt”.Various additives may be combined with the polymer before, during, orafter melting. Those additives may be solids that may or may not undergoa solid to liquid (melting) phase change, solids that may or may not gothrough a solid to gas (sublimation) phase change, liquids that may ormay not undergo a liquid to gas (vaporization) phase change, or gases.The high intensity mixing device is used to attempt to uniformlydisperse the additive within the host polymer or carrier material. Afteradditive dispersion is complete, the melt is discharged through ashaping device or “die” that is used to prepare the final shape of thecompound. To form the convenient pellet shape, the die typically isoutfitted with circular orifices through which the molten compoundflows. The circular orifices form continuous cylinders of the compoundthat are subsequently cut to form pellets.

One type of additive is a colorant which may be a dye or solid pigmentthat may be compounded into a base polymer to form a masterbatch ofcolorant in a carrier polymer. It is known to produce such colorantmasterbatches by addition of the colorant directly with the polymer,with or without premixing, into a Banbury mixer or a single or twinscrew extruder hopper and thereby into an extruder which mixes theadditive and carrier polymer together to form the masterbatch. Anotherknown method is to disperse pigment into a liquid carrier such asmineral oil and admit the dispersion into the extruder hopper. Also,solid additives such as minerals e.g. calcium carbonate, silica and thelike, and pigments, etc. as well as nonaqueous liquids such astackifiers, antioxidants, slip agents and antifog agents may beintroduced directly into a polymer melt to form an article ormasterbatch pellets. It has also been suggested to introduce water aloneinto a polymer melt to act as a foaming agent.

The quality and functionality of dispersed additives depends uponcertain characteristics of the additive and polymeric carrier as well asthe mixing device. In the case of dispersing solid additive particles ina polymeric carrier, the distribution of sizes of the particles has aprimary impact on the homogeneity of the resultant compound andfabricated articles that include that compound. The degree ofhomogeneity can, in turn, affect the functionality of the finishedarticle. Additionally, it is desirable that the configuration of themixing device and its conditions of use be appropriately selected suchthat the total surface area of the particles contacted by the carrierpolymer is maximized. It is generally regarded that the presence ofpoorly wetted particle agglomerations is undesirable. Therefore, thereis a need for a method of introducing additives into a polymer toprovide a homogeneous blend.

BRIEF SUMMARY

In various aspects, a process is provided for introducing an additive,particularly an inorganic salt, into a polymer melt to form a blend. Afilm may be formed from the polymer blend and incorporated into apackage for a food product.

In one aspect, the process includes introducing a polymer into anextruder. This polymer, which may be referred to as a base polymer orcarrier polymer or resin, is heated to a temperature sufficient to forma polymer melt. Generally this temperature will be above a melting pointof the base polymer, or above the glass transition temperature e.g. foramorphous polymers not having a melting point. A liquid, preferablywater, including an additive is introduced into the extruder. The liquidis mixed with the polymer melt in the extruder to form a blend. At leasta portion of the liquid vaporizes and vents from the extruder leaving atleast a portion of the additive intimately admixed with the polymer. Theadditive containing polymer blend is extruded.

In another aspect, a method of producing a plastic film suitable forforming packaging, e.g. for meat, includes introducing a base polymerinto an extruder. The base polymer is heated to a temperature sufficientto form a polymer melt e.g. above a melting point of the base polymer. Aliquid including an additive is introduced into the polymer melt. Theadditive containing liquid is mixed with the polymer melt in theextruder to form a blend. At least a portion of the liquid vaporizes andvents from the extruder. The blend of additive and polymer is extrudedand formed into a film or sheet. The film or sheet is incorporated intoa package, wherein the film or sheet forms a layer e.g. an interiorlayer of the package. An article such as a meat product is disposed inthe package.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of one embodiment of a process forintroducing an additive into a polymer melt.

FIG. 2 is a representation of the zones of an extruder.

DETAILED DESCRIPTION

The invention is described with reference to the drawings in which likeelements are referred to by like numerals. The relationship andfunctioning of the various elements of this invention are betterunderstood by the following detailed description. However, theembodiments of this invention as described below are by way of exampleonly, and the invention is not limited to the embodiments illustrated inthe drawings.

The invention is directed to a method of blending an additive to apolymer. In one aspect, it is directed to a method of introducing anadditive containing liquid, preferably an aqueous solution, dispersionor emulsion, into a polymer melt. In another aspect, it is directed to amethod of producing a homogeneous blend of a polymer including a salt.

In a preferred embodiment, the additive may be a myoglobin bloomingagent. A “myoglobin blooming agent” refers to any agent (or precursorthereof) that binds to or interacts with any myoglobin-containingstructure (including but not limited to deoxymyoglobin, oxymyoglobin,metmyoglobin, carboxymyoglobin, and nitric oxide myoglobin) present in afresh meat product to produce or preserve a desired color, such as a redcolor indicative of fresh meat. The myoglobin blooming agent may alsointeract or cause an interaction with hemoglobin present in a meatproduct so as to produce, maintain or enhance i.e. “fix” a desiredcolor. Thus, the myoglobin blooming agent is not a color additive, butit acts as a color fixative. In one preferred embodiment, the myoglobinblooming agent is a “nitric oxide donating compound” (“NO donor”) thatprovides a nitric oxide (NO) molecule that binds to the myoglobinpresent in a meat product so as to maintain or promote a reddening orblooming or other favorable coloration of the meat product. A nitricoxide donating compound releases nitric oxide or is a precursor e.g.nitrate which acts as an intermediate leading to the formation of nitricoxide which binds to a myoglobin molecule in a meat product. AnNO-donating compound comprises compounds capable of forming a nitrosylgroup. In one aspect, the myoglobin blooming agent is a nitrate (MNO₃)or nitrite (MNO₂) salt, where suitable metal counter ion (M⁺) can beselected from the group consisting of: alkali metals (e.g. sodium,potassium), alkaline earth metals (e.g. calcium), transition metals,ammonium and or protonated primary, secondary, or tertiary amines orquaternary amines. In another aspect, the myoglobin blooming agentcomprises a Fremy's salt [NO(SO₃Na)₂ and NO(SO₃K)₂]. Other suitablenitric oxide donating agents are disclosed in U.S. Pat. Nos. 6,706,274to Herrmann et al. (filed Jan. 18, 2001), 5,994,444 to Trescony et al.(filed Oct. 16, 1997), and 6,939,569 to Green et al. (filed Jun. 18,1999), as well as published U.S. Patent Application No. US2005/0106380by Gray et al. (filed Nov. 13, 2003). The myoglobin blooming agent maybe a salt, particularly a nitrite or nitrate salt. Sodium nitrate orsodium nitrite or blends thereof may typically be used. Potassiumnitrate or potassium nitrite may also be used. Additionally suitablecompounds may include a nitrogen containing agent that promotes therelease or formation of NO such as nitrite reductase, nitrate reductaseor nitrosothiol reductase catalytic agents, including the materialsdescribed in WIPO Publication No. WO 02/056904 by Meyerhoff et al.(filed Jan. 16, 2002), which is incorporated herein by reference. It isexpected that these agents and compounds would be suitable myoglobinblooming agents. Other suitable agents may include sulfur containingcompounds that similarly bind or act as precursors or intermediates toagents that fix a desirable color by binding to myoglobin.

Myoglobin blooming agents and solutions or dispersions thereof may becolorless or may be slightly colored. For example, sodium nitrite mayhave an intrinsic pale color (i.e. may not be totally colorless), butthis color does not typically have sufficient intensity itself to act asa significant colorant or color additive. However, this does notpreclude either the use of colored myoglobin blooming agents whichimpart an intrinsic color or the combination of a myoglobin bloomingagent in combination with one or more natural and/or artificialcolorants, pigments, dyes and/or flavorants such as annatto, bixin,norbixin, beet powder, caramel, carmine, cochineal, turmeric, paprika,liquid smoke, one or more FD&C colorants, etc.

A polymer film may be made from the polymer comprising a salt orNO-donating compound. This polymer film is particularly useful inpackaging for food products such as meat, where the NO donation compoundis used to produce a bloom in the meat.

In referring to film structure, a slash “/” will be used to indicatethat components to the left and right of the slash are in differentlayers and the relative position of components in layers may be soindicated by use of the slash to indicate film layer boundaries.Acronyms commonly employed herein include:

-   -   EAA—Copolymer of ethylene with acrylic acid    -   EAO—Copolymers of ethylene with at least one α-olefin    -   EBA—Copolymer of ethylene with butyl acrylate    -   EEA—Copolymer of ethylene with ethyl acrylate    -   EMA—Copolymer of ethylene with methyl acrylate    -   EMAA—Copolymer of ethylene with methacrylic acid    -   EVA—Copolymer of ethylene with vinyl acetate    -   EVOH—A saponified or hydrolyzed copolymer of ethylene and vinyl        acetate    -   PE—Polyethylene (an ethylene homopolymer and/or copolymer of a        major portion of ethylene with one or more α-olefins)    -   PP—Polypropylene homopolymer or copolymer    -   PET—Poly(ethylene terephthalate)    -   PVDC—Polyvinylidene chloride (also includes copolymers of        vinylidene chloride, especially with vinyl chloride and/or        methyl acrylate (MA)), also referred to as saran

A “core layer,” as used herein, refers to a layer positioned between andin contact with at least two other layers.

An “outer layer,” as used herein, is a relative term and need not be asurface layer.

The term “exterior layer” refers to a layer comprising the outermostsurface of a film or product.

The term “interior layer” refers to a layer comprising the innermostsurface of a film or product. For example, an interior layer forms theinterior surface of an enclosed package. The interior layer can be thefood-contact layer and/or the sealant layer.

As used herein, the term “barrier,” and the phrase “barrier layer,” asapplied to films and/or film layers, are used with reference to theability of a film or film layer to serve as a barrier to one or moregases or moisture.

The term “adhesive layer,” or “tie layer,” refers to a layer or materialplaced on one or more layers to promote the adhesion of that layer toanother surface. Preferably, adhesive layers are positioned between twolayers of a multilayer film to maintain the two layers in positionrelative to each other and prevent undesirable delamination. In someembodiments, a peelable tie layer may be used which is designed to haveeither cohesive failure or delamination from one or both adjacent layersupon application of a suitable manual force to provide an openingfeature for a package made from the film. Unless otherwise indicated, anadhesive layer can have any suitable composition that provides a desiredlevel of adhesion with the one or more surfaces in contact with theadhesive layer material. Optionally, an adhesive layer placed between afirst layer and a second layer in a multilayer film may comprisecomponents of both the first layer and the second layer to promotesimultaneous adhesion of the adhesive layer to both the first layer andthe second layer to opposite sides of the adhesive layer.

As used herein, the phrases “seal layer,” “sealing layer,” “heat seallayer,” and “sealant layer,” refer to a film layer, or layers, involvedin the sealing of the film: to itself; to another film layer of the samefilm or another film; and/or to another article which is not a film e.g.a tray. In general, the sealant layer is a surface layer i.e. anexterior or an interior layer of any suitable thickness, that providesfor the sealing of the film to itself or another layer. With respect topackages having only fin-type seals, as opposed to lap-type seals, thephrase “sealant layer” generally refers to the interior surface filmlayer of a package. The inside layer frequently can also serve as a foodcontact layer in the packaging of foods.

“Polyolefin” is used herein broadly to include polymers such aspolyethylene, ethylene-alpha olefin copolymers (EAO), polypropylene,polybutene, and ethylene copolymers having a majority amount by weightof ethylene polymerized with a lesser amount of a comonomer such asvinyl acetate, and other polymeric resins falling in the “olefin” familyclassification. Polyolefins may be made by a variety of processes wellknown in the art including batch and continuous processes using single,staged or sequential reactors, slurry, solution and fluidized bedprocesses and one or more catalysts including for example, heterogeneousand homogeneous systems and Ziegler, Phillips, metallocene, single siteand constrained geometry catalysts to produce polymers having differentcombinations of properties. Such polymers may be highly branched orsubstantially linear and the branching, polydispersity and averagemolecular weight may vary depending upon the parameters and processeschosen for their manufacture in accordance with the teachings of thepolymer arts.

“Polyethylene” is the name for a polymer whose basic structure ischaracterized by the chain —(CH₂—CH₂—)_(n). Polyethylene homopolymer isgenerally described as being a solid which has a partially amorphousphase and partially crystalline phase with a density of between 0.915 to0.970 g/cm³. The relative crystallinity of polyethylene is known toaffect its physical properties. The amorphous phase imparts flexibilityand high impact strength while the crystalline phase imparts a highsoftening temperature and rigidity.

Unsubstituted polyethylene is generally referred to as high densityhomopolymer and has a crystallinity of 70 to 90 percent with a densitybetween about 0.96 to 0.97 g/cm³. Most commercially utilizedpolyethylenes are not unsubstituted homopolymer but instead have C₂-C₈alkyl groups attached to the basic chain. These substitutedpolyethylenes are also known as branched chain polyethylenes. Also,commercially available polyethylenes frequently include othersubstituent groups produced by copolymerization. Branching with alkylgroups generally reduces crystallinity, density and melting point. Thedensity of polyethylene is recognized as being closely connected to thecrystallinity. The physical properties of commercially availablepolyethylenes are also affected by average molecular weight andmolecular weight distribution, branching length and type ofsubstituents.

People skilled in the art generally refer to several broad categories ofpolymers and copolymers as “polyethylene.” Placement of a particularpolymer into one of these categories of “polyethylene” is frequentlybased upon the density of the “polyethylene” and often by additionalreference to the process by which it was made since the process oftendetermines the degree of branching, crystallinity and density. Ingeneral, the nomenclature used is nonspecific to a compound but refersinstead to a range of compositions. This range often includes bothhomopolymers and copolymers.

For example, “high density” polyethylene (HDPE) is ordinarily used inthe art to refer to both (a) homopolymers of densities between about0.960 to 0.970 g/cm³ and (b) copolymers of ethylene and an alpha-olefin(usually 1-butene or 1-hexene) which have densities between 0.940 and0.958 g/cm³. HDPE includes polymers made with Ziegler or Phillips typecatalysts and is also said to include high molecular weight“polyethylenes.” In contrast to HDPE, whose polymer chains have somebranching, are “ultra high molecular weight polyethylenes” which areessentially unbranched specialty polymers having a much higher molecularweight than the high molecular weight HDPE.

Hereinafter, the term “polyethylene” will be used (unless indicatedotherwise) to refer to ethylene homopolymers as well as copolymers ofethylene with alpha-olefins and the term will be used without regard tothe presence or absence of substituent branch groups.

Another broad grouping of polyethylene is “high pressure, low densitypolyethylene” (LDPE). LDPE is used to denominate branched homopolymershaving densities between 0.915 and 0.930 g/cm³. LDPEs typically containlong branches off the main chain (often termed “backbone”) with alkylsubstituents of 2 to 8 or more carbon atoms.

Linear Low Density Polyethylene (LLDPE) are copolymers of ethylene withalpha-olefins having densities from 0.915 to 0.940 g/cm³. Thealpha-olefin utilized is usually 1-butene, 1-hexene, or 1-octene andZiegler-type catalysts are usually employed (although Phillips catalystsare also used to produce LLDPE having densities at the higher end of therange, and metallocene and other types of catalysts are also employed toproduce other well known variations of LLDPEs).

Ethylene α-olefin copolymers are copolymers having an ethylene as amajor component copolymerized with one or more alpha olefins such asoctene-1, hexene-1, or butene-1 as a minor component. EAOs includepolymers known as LLDPE, VLDPE, ULDPE, and plastomers and may be madeusing a variety of processes and catalysts including metallocene,single-site and constrained geometry catalysts as well as Ziegler-Nattaand Phillips catalysts.

Very Low Density Polyethylene (VLDPE) which is also called “Ultra LowDensity Polyethylene” (ULDPE) comprises copolymers of ethylene withalpha-olefins, usually 1-butene, 1-hexene or 1-octene and is recognizedby those skilled in the art as having a high degree of linearity ofstructure with short branching rather than the long side branchescharacteristic of LDPE. However, VLDPEs have lower densities thanLLDPEs. The densities of VLDPEs are recognized by those skilled in theart to range between 0.860 and 0.915 g/cm³. A process for making VLDPEsis described in European Patent Document publication number 120,503whose text and drawing are hereby incorporated by reference into thepresent document. Sometimes VLDPEs having a density less than 0.900g/cm³ are referred to as “plastomers”.

Polyethylenes may be used alone, in blends and/or with copolymers inboth monolayer and multilayer films for packaging applications for suchfood products as poultry, fresh red meat and processed meat.

As used herein, the term “modified” refers to a chemical derivative e.g.one having any form of anhydride functionality, such as anhydride ofmaleic acid, crotonic acid, citraconic acid, itaconic acid, fumaricacid, etc., whether grafted onto a polymer, copolymerized with apolymer, or otherwise functionally associated with one or more polymers,and is also inclusive of derivatives of such functionalities, such asacids, esters, and metal salts derived therefrom. Another example of acommon modification is acrylate modified polyolefins.

As used herein, terms identifying polymers, such as e.g. “polyamide” or“polypropylene,” are inclusive of not only polymers comprising repeatingunits derived from monomers known to polymerize to form a polymer of thenamed type, but are also inclusive of comonomers, as well as bothunmodified and modified polymers made by e.g. derivitization of apolymer after its polymerization to add functional groups or moietiesalong the polymeric chain. Furthermore, terms identifying polymers arealso inclusive of “blends” of such polymers. Thus, the terms “polyamidepolymer” and “nylon polymer” may refer to a polyamide-containinghomopolymer, a polyamide-containing copolymer or mixtures thereof.

The term “polyamide” means a high molecular weight polymer having amidelinkages (—CONH—)_(n) which occur along the molecular chain, andincludes “nylon” resins which are well known polymers having a multitudeof uses including utility as packaging films, bags, and casings. See,e.g. Modern Plastics Encyclopedia, 88 Vol. 64, No. 10A, pp 34-37 and554-555 (McGraw-Hill, Inc., 1987) which is hereby incorporated byreference. Polyamides are preferably selected from nylon compoundsapproved for use in producing articles intended for use in processing,handling, and packaging food.

The term “nylon” as used herein it refers more specifically to syntheticpolyamides, either aliphatic or aromatic, either in crystalline,semi-crystalline, or amorphous form characterized by the presence of theamide group —CONH. It is intended to refer to both polyamides andco-polyamides.

Thus the terms “polyamide” or “nylon” encompass both polymers comprisingrepeating units derived from monomers, such as caprolactam, whichpolymerize to form a polyamide, as well as copolymers derived from thecopolymerization of caprolactam with a comonomer which when polymerizedalone does not result in the formation of a polyamide. Preferably,polymers are selected from compositions approved as safe for producingarticles intended for use in processing, handling and packaging of food,such as nylon resins approved by the U.S. Food and Drug Administrationprovided at 21 CFR § 177.1500 (“Nylon resins”), which is incorporatedherein by reference. Examples of these nylon polymeric resins for use infood packaging and processing include: nylon 66, nylon 610, nylon66/610, nylon 6/66, nylon 11, nylon 6, nylon 66T, nylon 612, nylon 12,nylon 6/12, nylon 6/69, nylon 46, nylon 6-3-T, nylon MXD-6, nylon MXDI,nylon 12T and nylon 6I/6T disclosed at 21 CFR § 177.1500. Examples ofsuch polyamides include nylon homopolymers and copolymers such as thoseselected form the group consisting of nylon 4,6 (poly(tetramethyleneadipamide)), nylon 6 (polycaprolactam), nylon 6,6 (poly(hexamethyleneadipamide)), nylon 6,9 (poly(hexamethylene nonanediamide)), nylon 6,10(poly(hexamethylene sebacamide)), nylon 6,12 (poly(hexamethylenedodecanediamide)), nylon 6/12 (poly(caprolactam-co-dodecanediamide)),nylon 6,6/6 (poly(hexamethylene adipamide-co-caprolactam)), nylon 66/610(e.g., manufactured by the condensation of mixtures of nylon 66 saltsand nylon 610 salts), nylon 6/69 resins (e.g., manufactured by thecondensation of epsilon-caprolactam, hexamethylenediamine and azelaicacid), nylon 11 (polyundecanolactam), nylon 12 (polylauryllactam) andcopolymers or mixtures thereof.

In use of the term “amorphous nylon copolymer,” the term “amorphous” asused herein denotes an absence of a regular three-dimensionalarrangement of molecules or subunits of molecules extending overdistances which are large relative to atomic dimensions. However,regularity of structure may exist on a local scale. See, “AmorphousPolymers,” Encyclopedia of Polymer Science and Engineering, 2nd Ed., pp.789-842 (J. Wiley & Sons, Inc. 1985). In particular, the term “amorphousnylon copolymer” refers to a material recognized by one skilled in theart of differential scanning calorimetry (DSC) as having no measurablemelting point (less than 0.5 cal/g) or no heat of fusion as measured byDSC using ASTM 3417-83. The amorphous nylon copolymer may bemanufactured by the condensation of hexamethylenediamine, terephthalicacid, and isophthalic acid according to known processes. Amorphousnylons also include those amorphous nylons prepared from condensationpolymerization reactions of diamines with dicarboxylic acids. Forexample, an aliphatic diamine is combined with an aromatic dicarboxylicacid, or an aromatic diamine is combined with an aliphatic dicarboxylicacid to give suitable amorphous nylons.

As used herein, “EVOH” refers to ethylene vinyl alcohol copolymer. EVOHis otherwise known as saponified or hydrolyzed ethylene vinyl acetatecopolymer, and refers to a vinyl alcohol copolymer having an ethylenecomonomer. EVOH is prepared by the hydrolysis (or saponification) of anethylene-vinyl acetate copolymer. The degree of hydrolysis is preferablyfrom about 50 to 100 mole percent, more preferably, from about 85 to 100mole percent, and most preferably at least 97%. It is well known that tobe a highly effective oxygen barrier, the hydrolysis-saponification mustbe nearly complete, i.e. to the extent of at least 97%. EVOH iscommercially available in resin form with various percentages ofethylene and there is a direct relationship between ethylene content andmelting point. For example, EVOH having a melting point of about 175° C.or lower is characteristic of EVOH materials having an ethylene contentof about 38 mole % or higher. EVOH having an ethylene content of 38 mole% has a melting point of about 175° C. With increasing ethylene contentthe melting point is lowered. Also, EVOH polymers having increasing molepercentages of ethylene have greater gas permeabilities. A melting pointof about 158° C. corresponds to an ethylene content of 48 mole %. EVOHcopolymers having lower or higher ethylene contents may also beemployed. It is expected that processability and orientation would befacilitated at higher contents; however, gas permeabilities,particularly with respect to oxygen, may become undesirably high forcertain packaging applications which are sensitive to microbial growthin the presence of oxygen. Conversely, lower contents may have lower gaspermeabilities, but processability and orientation may be moredifficult.

As used herein, the term “polyester” refers to synthetic homopolymersand copolymers having ester linkages between monomer units which may beformed by condensation polymerization methods. Polymers of this type arepreferable aromatic polyesters and more preferable, homopolymers andcopolymers of poly(ethylene terephthalate), poly(ethylene isophthalate),poly(butylene terephthalate), poly(ethylene naphthalate) and blendsthereof. Suitable aromatic polyesters may have an intrinsic viscositybetween 0.60 to 1.0, preferably between 0.60 to 0.80.

Turning now to a first aspect of the invention, a method is provided forintroducing an additive into a polymer melt. The additive may be aliquid, especially a viscous liquid, but is preferably a solid at roomtemperature (˜23° C.). The liquid is a liquid at room temperature andmay be any suitable liquid which is selected depending upon the choiceof a variety of parameters including e.g. additive solubility and/ordispersibility, safety, cost, available equipment for processing,compatibility with the carrier polymer, intended uses, etc. Suitableliquids may include polar or nonpolar solvents. Non-oil based andsubstantially oil free (especially mineral oil free) liquid compositionsare preferred. Water, alcohols or mixtures thereof are preferred assolvents or liquids. Water is especially preferred. Solutions arepreferred over dispersions and emulsions. Aqueous solutions of adissolved additive solute are especially preferred. It is furthercontemplated that “liquid” may include addition of additive and solidwater i.e. ice under conditions which may create in situ an additivecontaining aqueous solution, dispersion or emulsion.

Thus, a liquid comprising an additive e.g. an additive solute of e.g. asalt dissolved in water is introduced into a melt of a base polymer. Thequality of the mixing, and functionality of the resulting blend, maydepend upon the physical and chemical properties of the additive and thebase polymer as well as process parameters such as the type andconfiguration of mixing equipment. It is desirable to achieve goodmixing of the liquid and the polymer melt for uniform dispersion of theadditive within the melt.

A schematic of one embodiment of a process is shown in FIG. 1. Anextruder 10 is provided. The extruder is fed a polymer from a polymersource 12. The polymer is generally provided in pellet form. Theextruder is also fed a liquid 14 comprising water and an additive. Thelocation of the liquid injection may vary and will be described in moredetail below. From the extruder 10, the molten polymer including theadditive is fed to a die 16, where it is pelletized. The pellets providea masterbatch 18 of the base polymer blended with the additive. Themasterbatch 18 may then be added to more of the base polymer in a mixer20 and fed into a second extruder 22. The second extruder may then beused to form a film 24.

Turning now to the extrusion process, aspects of an extrusion processare well known in the art. In extrusion, plastic pellets or granules areplasticized, homogenized, and continuously or intermittently formed intoarticles. The extrusion process can be combined with a variety ofoperations some of which may be applied after extrusion as known in theart. Such operations include film or sheet forming, tubular filmforming, orientation, blow molding, thermoforming, injection molding,rotational molding, compression molding, foaming, uniaxial or biaxialstretching, calendaring, machining, and punching. A variety of types ofextruders can be used, including single screw, twin screw, andmultiscrew extruders. The extruder typically includes the followingelements: a feed hopper into which plastic pellets are charged; abarrel, which contains the screw; one or more screws, which plasticates,heats, fluidizes, homogenizes, and/or transports the plastic to the die;a screen pack and breaker plates for filtration of the polymer melt andproviding back pressure; and a die for establishing the extrusionprofile.

A particular embodiment of an extruder that may be used in the processis shown in FIG. 2. The extruder is a twin screw extruder. It is dividedinto a variety of zones 31-41. The base polymer is fed into the feedzone 31. The base polymer may be fed through a feed port in feed zone31. Heating and melting of the polymer occurs in zone 32. Paddles areprovided in zone 33 for intensifying the mixing. A restriction devicesuch as a full bore orifice plug is provided in zone 34. The restrictiondevice fills most of the area of the barrel and the polymer melt isforced around the restriction device. This restriction causes the mixingsections upstream of the restriction to completely fill with the polymeror polymer/additive blend. The restriction also tends to increase theresidence time of the material in the mixing zone and consequentlyimprove melting, additive dispersion and other qualities that benefitfrom mixing. Other restriction devices besides the full bore orificeplug may be used, depending on the type of extruder. A high free volumeor reduced pressure area is provided in zone 35. The liquid, whichincludes preferably water and the additive, is injected in zone 36. Theliquid may be injected through an injection port, which is located at aposition downstream of the base polymer feed port. The polymer/additivecombination moves into an intensive mixing zone 37 where furtherdispersion of the additive into the base polymer occurs. An additionalrestriction device or plug is provided in zone 38. The polymer melt isabove the boiling point of the liquid (the following description willrefer to water as the liquid, however it will be understood that otherliquids may also be used in like manner), therefore portions of theliquid e.g. water begin to vaporize after the injection. This vaporizedwater exits the extruder through a vent in zone 39. The vent may belocated at any suitable position downstream from the liquid injectionport e.g. in a section having a high free volume or reduced pressure toavoid solids exiting through the vent (hereinafter termed “vent creep”).The vent may be open to the environment or it may be connected to apumping device such as a vacuum pump. The vapor may also vent at otherlocations in the extruder, such as the inlet port in zone 31. Furthermixing occurs at zone 40, and the melt exits the extruder at zone 41. Ingeneral, at least two restriction devices such as full bore orificeplugs may be provided e.g. one downstream of the additive solution feedport and one upstream of the solution feed port to create an area ofconfinement for liquid water. However, it is possible to run the processwithout a restriction device such as a plug.

It has been found that the temperature of the polymer melt may have thefollowing effect on the processing; if the temperature is too high, thelower viscosity of the melt may promote vent creep which can forcematerial out of the vent. This vent creep may be exacerbated by thepresence of processing additives within the base polymer. It has beenfound that heating the base polymer to a temperature just above themelting point of the base polymer minimizes the amount of vent creep dueto the addition of the solution. For polyethylene, the polymer may beheated to a temperature of above about 275° F. (135° C.), preferably toabout 330° F. (166° C.). For various materials, the polymer ispreferably heated to a temperature of less than 100° F. above themelting point of the polymer.

The process depicted in FIG. 2 has several beneficial aspects. Becausethe additive is preferably introduced into an already molten carrierpolymer, the residence time at high temperature and high shear isreduced. However, the additive may also be introduced before the polymeris melted. The evaporating water also removes some of the energy fromthe system, further reducing the exposure temperature of the additive.This is an advantage for the generation of masterbatches containingthermally sensitive materials. By providing for the addition of amaterial as a solution or suspension, the present method allows for thecreation of low average particle size, narrow distributions of materialsin polymers. In instances where the dispersed material is unavailable asa fine particle size powder, this technique may be used to create highquality dispersions. Dispersions or emulsions utilizing the inventivemethod may beneficially have a desirable uniformity of distribution ofthe additive throughout the base polymer which in turn may be used toprovide polymeric articles incorporating the highly uniform additivecontaining masterbatch to produce highly uniform polymeric articles orarticle components such as a layer in a multilayer thermoplastic filmhaving a highly uniform distribution of additive. Advantageously,solutions according to the present invention which have an additivesolute dissolved in a solvent such as water may have the aboveuniformity of distribution benefit coupled with an additional benefit ofuniformity of particle size of the additive. It is believed that as theheat of the polymer melt drives off the solvent as vapor from the wellmixed solubilized additive containing liquid and polymer combination,that the additive forms small uniform particles. This uniformity ofparticle size delivers a corresponding uniformity of functionalityacross the polymer when coupled with the uniformity of distributionwhich also results from the inventive process. In dispersed solid inliquid dispersion embodiments of the invention, the particle size ismore dependent upon the ingoing particle size since the particlesthemselves are already solid rather than being formed in situ uponvaporization. Nonetheless the process of the invention also provides anadvantageous uniform distribution of dispersed particles even if theaverage particle size remains relatively unchanged in a dispersionembodiment compared to a solution embodiment.

A variety of additives can be added to a base polymer in this manner.The high temperature and shearing conditions may tend to degrade somematerials, so that may limit the types of materials that may be used asan additive. As noted above, the additive may be either dissolved orsuspended in a liquid such as water. Thus, possible additives includesoluble additives and especially water soluble additives such as salts,as well as water suspensions such as colloids, foams, emulsions,dispersions and sols. A variety of different types of materials may beadded in order to produce a polymer with certain desired properties. Inone embodiment, the additive may be a colorant, opacifier, flavorant,perfume, anti-microbial, fungicide, antioxidant, protein, enzyme,antiblocking agent, antistatic agent, antifog agent, slip agent, lightstabilizer, light absorber, process aid, release agent, a reactiveindicator compound, or taggants e.g. rare earth elements or markercompositions. The additive may be an element or a compound and either asingle material or a blend of materials.

In one embodiment, the additive is a salt with a cation and an anion.The cation may be selected from the following group: ammonium, lithium,sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium,barium, strontium, aluminum, iron II, iron III, cobalt, nickel, copperI, copper II, zinc, silver, and tin. The anion may be selected from thefollowing group: halide (e.g., fluoride, chloride, bromide, iodide,astatide), oxide, sulfide, phosphate, hydrogen phosphate, dihydrogenphosphate, pyrophosphate, polyphosphate, sulfate, hydrogen sulfate,pyrosulfate, sulfite, hydrogen sulfite, pyrosulfite, thiosulfite,thiosulfate, carbonate, hydrogen carbonate, tetraborate, metaborate,nitrate, nitrite, hydroxide, and silicate. The additive may also includesoluble polymers, acids, or organic acids (such as propionic acid,formic acid, acetic acid, benzoic acid, and sorbic acid) or the salts ofthose acids. Inorganic salts such as alkali metal nitrites and nitratesare preferred additives e.g. for delivery of myoglobin blooming agentsto a meat surface via a polymeric food package in vacuum package or toprovide binding agents to reactive sites to affect other functionalitiesin nonfood applications such as surface oxidation prevention.

In one embodiment, the polymer blend including the additive preferablyincludes less than 1% (by weight of the polymer) of benzodiazole,benzotriazole, amine benzoates, amine molybdates, amine nitrates, sodiumbenzoate, or sodium molybdate.

In a particular embodiment, the additive is a myoglobin blooming agentsuch as an NO donating compound. This is particularly useful for meatpackaging. It is desirable for meat to have the red or “bloomed” meatcolor that consumers use as their primary criterion for perceivingfreshness. NO donating compounds, such as sodium nitrite or sodiumnitrate, provided on the surface of an interior layer of packagingaffect the color of vacuum packaged meat. In particular, when sodiumnitrite is coated onto the inner contact film surface of a vacuumpackage, the meat may turn brown after evacuating oxygen. After a periodof time, the preferred red color gradually displaces the brown color andremains stable in a vacuum package for several months. See e.g. WO2005/097486 (Siegel) published on Oct. 20, 2005 and Pockat et al U.S.patent application entitled, “Myoglobin Blooming Agent Containing ShrinkFilms, Packages and Methods for Packaging” filed Apr. 20, 2006, Ser. No.______, both of which whose teachings are hereby incorporated byreference in their entireties.

Turning now to packaging, packaging can consist of a variety of layersincluding monolayer polymeric structures such as films or trays as wellas multilayer structures such as films, trays or other articles of 2, 3,4, 5, 6, 7, 8, 9, or more layers. In one embodiment, multilayerpackaging films are provided that comprise a first layer and a secondlayer positioned in any suitable configuration. The first layer ispreferably a heat-resistant layer and the second layer is preferably asealant layer.

Examples of various preferred multilayer configurations include thefollowing:

Abuse Resistant (Exterior)/Barrier/Sealant (Interior);

Abuse Resistant (Exterior)/Core/Barrier/Core/Sealant (Interior);

Abuse Resistant (Exterior)/Adhesive/Core/Barrier/Core/Sealant(Interior);

Abuse Resistant (Exterior)/Adhesive/Core/Barrier/Core/Adhesive/Sealant(Interior);

Abuse Resistant (Exterior)/Adhesive/Barrier/Adhesive/Sealant (Interior);

Abuse Resistant (Exterior)/Nylon Core/Barrier/Core/Sealant (Interior);

Abuse Resistant (Exterior)/Adhesive/Core/Barrier/Nylon Core/Sealant(Interior); and

Abuse Resistant (Exterior)/Adhesive/Core/Barrier/NylonCore/Adhesive/Sealant (Interior)

A brief description of each layer follows.

Abuse-Resistant Outer Layer

In one aspect, the multilayer film can include an abuse resistant layer(which may also be a heat resistant layer) that can include a polyolefinsuch as polyethylene or polypropylene, ionomer, polyester, nylons suchas semi-crystalline or amorphous nylon or blends thereof e.g. a blend ofan amorphous nylon copolymer, a low temperature polyamide and/or a hightemperature polyamide. A heat resistant layer can be positioned at ornear the exterior surface of the packaging film.

Barrier Layers

The multilayer packaging films can further include a barrier layer,which is preferably a gas barrier layer. The gas barrier layer ispreferably an oxygen barrier layer, and is preferably a core layerpositioned between the exterior and interior layers. The barrier layercan comprise any suitable material, such as EVOH, PVDC,polyacrylonitrile, nylon, nanocomposite or a metal foil such asaluminum.

For perishable food packaging, the oxygen (O₂) permeability desirablyshould be minimized: Typical oxygen barrier films will have an O₂permeability of less than about 310 cm³/m² for a 24 hour period at 1atmosphere, 0% relative humidity and 23° C., and preferably less than 75cm³/m², more preferably less than 20 cm³/m². Barrier resins such as PVDCor EVOH in the core layer may be adjusted by blending in compatiblepolymers to vary gas permeability e.g. O₂ of the films.

Tie Layers

In addition to the first layer and the second layer, a multilayerpackaging film can further include one or more adhesive layers, alsoknown in the art as “tie layers,” which can be selected to promote theadherence of adjacent layers to one another in a multilayer film. Theadhesive layer is preferably formulated to aid in the adherence of onelayer to another layer by virtue of the compatibility of the materialsin that layer to the first and second layers. Peelable tie layers mayalso be utilized for provision of easy opening features for manualopening of a package.

Sealant/Food Contact Layer

The multilayer film can also include a sealant layer, or for foodpackaging a food contact layer, that is preferably positioned at or nearthe interior surface of the packaging film. In one preferred embodimentof the invention, the additive of the present invention may be providedin the sealant or interior layer to provide functionality to productscontained within a package. The sealant layer can comprise a suitableheat-sealable polymer such as an ethylene-α-olefin copolymer or ionomer.The sealant layer is preferably formulated and positioned to form a heatseal. Hermetically sealed packages may be made utilizing masterbatchesmade according to the present invention.

A sealant layer preferably comprises a heat sealable polymeric materialsuch as polyolefins including polypropylene homopolymers, polypropylenecopolymers, very low density polyethylene (VLDPE), ultra low densitypolyethylene (ULDPE), linear low density polyethylene (LLDPE) orhomogeneous polyolefin resins, such as made with single-site catalysts(SSC) e.g. metallocene or constrained geometry catalysts. Ethylene vinylacetate (EVA) copolymers are also suitable materials for forming theinner surface heat sealable layer. A sealant layer may also comprise anionomer such as Surlyn®, available from DuPont Company. This material isessentially a metal salt neutralized copolymer of ethylene and anorganic acid or acids like acrylic or methacrylic acid. Sealant layercan be 5 to 50% of the thickness of the total structure with a preferredthickness being about 15% of the total thickness. Preferred examples ofsuch sealable resins constituting a sealant layer may include:SSC-LLDPE, SSC-VLDPE, LLDPE, VLDPE, ULDPE, EVA, EMAA, EAA, EMA, andionomer resins. Examples of suitable resins include those sold by DowChemical Company under the trade names of “AFFINITY,” “ATTANE” and“ELITE” and those available from ExxonMobil Co. under the trade name of“EXACT” and “ESCORENE.”

Film Thickness

Preferably, the film has a total thickness of less than about 10 mils,more preferably the film has a total thickness of from about 1 to 10mils, still more preferably from about 1 to 5 mils, and yet still morepreferably, from about 1.5 to 3 mils. For example, entire single ormultilayer films or any single layer of a multilayer film can have anysuitable thicknesses, including 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 mils,or any increment of 0.1 or 0.01 mil therebetween.

At least one layer of packaging preferably includes an additive. Theadditive may be a salt and for certain preferred embodiments ispreferably an NO donating compound. The NO donating compound providesnitric oxide gas as a result of the reduction of the nitrite (or otherNO donating compound) on the package after contact with certain productssuch as meat and this gas affects the color development of the meat foodproduct under reduced oxygen conditions. The NO donating compound ispreferably incorporated within the interior layer. The NO donatingcompound is preferably evenly dispersed throughout the entire layer toenable any length of film incorporating the layer to includeapproximately similar amounts of the compound within the sealing layerfor a uniform effect on a food product. The NO donating compound ispreferably present not only in a uniform distribution, but also withuniformly small particle sizes i.e. a small particle size range of smallparticles.

In one embodiment, the NO donating compound may be selected frominorganic or organic nitrates, inorganic or organic nitrites,nitrosodisulfonates such as Fremy's salt, organic nitro compounds,O-nitrosylated compounds, S-nitrosylated compounds, N-nitrosylatedcompounds, nonoate compounds, transition metal/nitroso complexes,furoxans, sydnonimines, and oximes. The NO donating compound may be asalt, particularly an inorganic salt, more particular a nitrite ornitrate salt. Sodium nitrate or sodium nitrite may typically be used.Potassium nitrate or potassium nitrite may also be used.

In another embodiment additives may comprise other types of myoglobinblooming agents such as nitrogen heterocycles e.g. niacin, andnicotinamide.

The base polymer can be any suitable polymer, and is typically apolyolefin. For example, the base polymer may be a polyethylene, e.g.very low density polyethylene (VLDPE), linear low density polyethylene(LLDPE), or low density polyethylene (LDPE). The base polymer may alsobe polypropylene, ethylene vinyl acetate, EMAA, and mixtures thereof.Particular examples include Dow ATTANE® 4201-G VLDPE, Dow 608A LDPE, DowAFFINITY, ExxonMobil ESCORENE, and Voridian 74556 LLDPE.

Therefore, for a water soluble material, it has been found thatproviding the additive as an aqueous solution provides a more uniformand consistent particle size and a more uniform distribution anddispersion of the compound within the polymer. The additive may be asalt or an NO donating compound. An aqueous solution is prepared fromthe additive. The concentration of the additive is preferably close tothe saturation concentration of the aqueous solution. For sodiumnitrite, the solution preferably includes between about 20 wt % andabout 42 wt % of the additive. The aqueous solution including theadditive and water is introduced into a polymer melt. This is typicallyperformed in an extruder to attempt to uniformly disperse the additivewithin the base polymer. The base polymer is preferably at a temperatureabove a melting point of the base polymer so as to form a polymer melt.It has been found that heating the base polymer to a temperature justabove the melting point of the base polymer minimizes the amount of ventcreep due to the addition of the solution. For polyethylene, the polymermay be heated to a temperature of above about 275° F. (135° C.),preferably about 330° F. (166° C.). The additive containing liquid ismixed with the polymer melt in the extruder to form a blend. At least aportion of the liquid (typically water) vaporizes and vents from theextruder. Suitably at least 10 wt %, preferably at least 50 wt. %, morepreferably at least 90 wt. %, and most preferably at least 98% of theliquid, preferably water, is removed from the base polymer and additivecontaining liquid blend prior to extrusion e.g. by vaporization of thewater and venting with or without a vacuum assist. The liquid reduced(thermally dewatered) blend is then extruded from the extruder. Theblend is then typically extruded into pellets, but may be extrudeddirectly as a film. The opening through which a polymer melt extrudatenormally exits the extruder and to which a die is normally attached isreferred to herein as a die regardless of whether a separate die isattached to the extrudate exit orifice or opening.

The NO donating compound or other additive may be added to the sameextruder used to form a polymeric structure such as a flexiblethermoplastic film. More commonly, the additive is first mixed with basepolymer to form a masterbatch of pelletized blends. Pellets from themasterbatch are convenient for subsequent use in fabricating articles.Thus, after dispersion of an additive such as an NO donating compoundwith a carrier polymer in a mixing device is complete, the polymer meltis discharged through a shaping device or die that is used to preparepellets of the masterbatch. To form the convenient pellet shape, the dietypically is outfitted with circular orifices through which the moltencompound flows, exiting the die and being solidified by cooling. Thecircular orifices form continuous cylinders or strands of the compoundthat are subsequently cooled and cut to form thermoplastic pellets.Pellets from the masterbatch may then be mixed with additional basepolymer or another polymer, heated to a plastic state and formed into adesired shape e.g. a film by any of the well known film formingprocesses. When used to create a masterbatch, a sufficient amount of thesolution is introduced into the polymer melt to obtain a blend thatincludes up to 10 wt. % or more of additive. For certain embodimentsadditive salts such as nitrite may beneficially be made having betweenabout 2 wt % and about 10 wt % additive, preferably between about 4 wt %and about 6 wt % additive. It will be recognized in view of the presentteachings that the upper limit may vary depending upon selection ofvarious parameters including additive and liquid composition andproperties such as solubility as well as equipment configuration andcapacities. The lower limit is that deemed practical for the intendeduse.

Exemplary of a suitable use for the invention, as previously described,the pellets from the masterbatch may be added to pellets of the basepolymer (and/or one or more different polymers) and then formed into anarticle, such as a tray, film or sheet. For example, to produce anarticle suitable for use in delivery of a myoglobin blooming agent to ameat surface in a package a sufficient amount of a solution may beintroduced into a polymer melt to obtain a polymer sheet includingbetween about 0.05 wt % and about 5 wt % salt, preferably between about0.1 wt % and about 2.5 wt % salt. The blended polymer sheet may then beincorporated into a package for a food product, particularly a meatproduct. The blended polymer sheet typically forms an inner layer of thepackage. The package may be used with any type of meat, including butnot limited to beef, pork, lamb, poultry, and fish. The desiredconcentration of NO donating compound may depend on the type of meatbeing packaged, the amount of available myoglobin, desired depth ofpenetration of agent into the meat surface, subsequent film formationoperations such as stretching which affect e.g. the surface area of thefilm, and other factors.

Without intending to be bound by theory, it is believed that nitricoxide gas forms as a result of the reduction of the nitrite (or other NOdonating compound) on the package and this gas affects the color of themeat food product. The nitric oxide gas is believed to have a similareffect on bloom as oxygen or carbon monoxide gas. The “bloom time” forthe red color to fully develop depends on the oxygen exposure as well asthe freshness of the muscle and the specific cut. A visually perceptible“well done” indication of cooking for the food product is hard toachieve when the nitric oxide gas penetrates intact muscle or groundmeat to depths that almost reach the center of the individual portion.Therefore, it is desirable to control the level of additive utilized toachieve a very shallow penetration by the color effect (believed to becaused by nitric oxide penetration) of the viewing surface of the foodproduct. As the depth of the nitric oxide gas penetration increases, theinternal color is less affected by cooking temperatures that normallyturn the color brown or grey. When this happens, a condition known as“pink ring” or “persistent pinking” may occur. See, McGee, “Meat,” OnFood and Cooking. Rev. Ed., 2004, Chapter 3, pp. 118-178, at p. 149(Scribner, New York, N.Y.) which chapter is hereby incorporated byreference in its entirety. It is not possible to cook the product to thenormal appearance of a well-done level. The packaging preferablyprovides between 0.01 to 10 μmoles, more preferably between 0.02 to 2μmoles, of the blooming agent per square inch to the uncooked meatproduct surface within 96 hours after contact.

Turning now to the formation of the packaging film, a monolayer ormultilayer film may be made by conventional processes including e.g.slot cast or blown film processes. It may also be made by athermoforming or by an orientation process, e.g. under conditions toproduce a film which is heat shrinkable at 90° C. or less. Descriptionsof suitable orientation processes are disclosed in U.S. Pat. No.5,759,648 to Idlas, which is hereby incorporated by reference in itsentirety.

The multilayer films and food packages may be manufactured bycoextrusion of all layers simultaneously, for example, as described inpublished U.S. Pat. Application No. 2004/0166262 to Busche et al.,entitled “Easy open heat-shrinkable packaging,” and incorporated hereinby reference in its entirety. Busche et al. also describes easy openingpeelable packaging. Other manufacturing methods are well known anddisclosed e.g. in U.S. Pat. No. 4,448,792 (Schirmer), or U.S. Pat. No.3,741,253 (Brax et al.), which discloses a coating lamination procedureto form a relatively thick primary multilayer extrudate either as a flatsheet or, preferably, as a tube which may be subsequently formed into afilm.

As generally recognized in the art, resin properties may be furthermodified by blending two or more resins together and it is contemplatedthat various resins may be blended into individual layers of themultilayer film or added as additional layers, such resins includeethylene-unsaturated ester copolymer resins, especially vinyl estercopolymers such as EVAs, or other ester polymers, very low densitypolyethylene (VLDPE), linear low density polyethylene (LLDPE), lowdensity polyethylene (LDPE), high density polyethylene (HDPE), nylons,ionomers, polypropylenes, polyesters, or any polymer listed in thisapplication or any document referenced herein or blends thereof. Theseresins and others may be mixed by well known methods using commerciallyavailable tumblers, mixers or blenders. Also, if desired, well knownadditives such as processing aids, slip agents, antiblocking agents,pigments, etc., and mixtures thereof may be incorporated into the film.

Various polymer modifiers may be incorporated for the purpose ofimproving or achieving particular properties for a desired applicationincluding for example toughness, orientability, and/or extensibility ofthe film or to affect optical properties such as gloss, transparency,haze, puncture resistance, abuse resistance, heat sealability,flexibility, etc. Other modifiers which may be added include: modifierswhich improve low temperature toughness or impact strength, andmodifiers which reduce modulus or stiffness. Exemplary modifiersinclude: styrene-butadiene, styrene-isoprene, and ethylene-propylenecopolymers.

All percentages reported herein are by weight unless otherwiseindicated.

EXAMPLES Example 1

A 40% solids sodium nitrite solution was prepared by dissolving 8 kg ofRepauno Products NaNO₂ (between 99.5%-99.7% pure and including about0.05% sodium carbonate and 0.2% sodium nitrate) (obtained from HydriteChemical Company, Brookfield, Wis.) in 12 kg of water. The solution wasmade with tap water at room temperature by gently agitating thewater/nitrite mixture.

Dow ATTANE® 4201-G VLDPE (obtained from Dow Chemical Company, Midland,Mich.) was loaded into the hopper of a gravimetric dosing unit that waspositioned to feed the polymer into the main feed port of an APVExtrusion Systems MP 2050 50 mm corotating twin screw extruder. Thefeeder was configured to dose the ATTANE at a rate of 41 kg/h. Themixing elements of the twin screw extruder were arranged in a fashionthat allowed for feeding and melting of the VLDPE, injection and mixingof the water/nitrite solution, removal of the water, pressurization of adie and formation of continuous strands of a homogeneous VLDPE/nitriteblend.

The twin screw extruder was electrically heated so that the feed zonewas at about 200° F. and the rest of the extruder at about 330° F. Whenthe extruder zones achieved the intended temperatures, the drive motorwas engaged to rotate the extruder screws at about 578 RPM. The ATTANEVLDPE was dosed into the primary feed port at about 41 kg/h. Once astable, homogeneous extrudate was achieved, the nitrite/water mixturewas injected into the molten VLDPE at an injection port. A gear pump wasused to deliver the nitrite/water solution to the injection port. Theinjection point was placed in a section of the extruder configured tohave high free volume and low pressure. The rate of delivery of thesolution was calculated by the time change in mass of the water/nitritemixture. To achieve the intended concentration of 5%, the pump speed wasset at 33 RPM. Then, the water/nitrite delivery rate was found to beabout 5.4 kg/h and the nitrite concentration, upon accounting for theremoval of the water, was 5%.

The mixing elements of the extruder were arranged in a fashion such thatthe liquid water/nitrite solution was prevented from moving upstream tothe primary feed port. Full bore orifice plugs were used to prevent theunwanted upstream migration.

Following injection, the nitrite water solution rapidly increased intemperature. The water fraction of the solution evaporated andeventually boiled. The resultant steam escaped through an atmosphericpressure vent port. It was noted that some steam also escaped throughthe primary feed port. Following this mixing section, the VLDPE/saltblend moved into a pressurization section and finally, into an eighthole strand die. Upon exiting the die, the resultant continuous strandswere cooled in a water bath. At the exit of the water bath, an air kniferemoved some of the moisture clinging to the surface of the stands.After leaving the influence of the air knife, the strands where cut intodiscrete pellets by a rotating knife-style pelletizer. Those pelletswere subsequently dried in a convection oven at about 50° C. (to removethe remainder of the surface moisture contributed by the water bath),packed in aluminum foil containing bags and stored for use.

Examples 2-5

Dow AFFINITY PL 1850 polyolefin plastomer (density=0.902 g/cm³; meltindex=3 dg/min) was dry blended with the masterbatch pellets produced inExample 1 by tumble mixing to yield net sodium nitrite contents of 0.1%,0.2%, 0.5%, and 1.0% by weight. Films were prepared from theplastomer/masterbatch pellet blends using a laboratory-scale cast filmextrusion line. The extruded films were about 8 inches wide and 2.25mils thick and exhibited excellent gloss and transparency. Each film waslaminated to an oxygen barrier layer-containing film of the followingcomposition: PETG/tie/nylon 6/EVOH/nylon 6/tie/VLDPE. The PETG was acopolymer of terephthalic acid, ethylene glycol and cyclohexanedimethanol. EVOH provided a high barrier to oxygen. The sodiumnitrite-containing cast films were joined to the VLDPE surface of theoxygen barrier film via thermal lamination where heat and pressure wasused to fuse the adjacent layers. The laminated films were fabricatedinto open pouches such that the nitrite-containing cast films formed theinnermost layer of the pouch. Fresh cut beef samples were inserted intothe pouches, the headspace oxygen was substantially removed and thepackages were closed with a heat seal. During refrigerated storage, thecolor of the packaged beef was observed over a period of time. After 72hours, the beef exhibited the results shown in Table 1 below. TABLE 1Sodium nitrite level Blooming Example 2 0.1% poor to none Example 3 0.2%poor to none Example 4 0.5% some bloom Example 5 1.0% excellent bloomIt can be seen from the results in Table 1, that at levels as low as0.5% sodium nitrite, the packaging provided a suitable bloom to thebeef. The bloomed color remained stable at least 30 days beyond the timeof packaging. It is believed that meat products having lower amounts ofmyoglobin such as pork, or poultry may show desirable bloom at lowerlevels of additive than that required for beef.

Comparative Example 6

A masterbatch was made as described in Example 1 except as indicatedbelow. A masterbatch of 10% sodium nitrite in low density polyethylenewas produced by simultaneously adding sodium nitrite powder (not insolution) and LDPE pellets to the feed section of the same corotatingtwin screw extruder used in Example 1. The LDPE was melted and thepowder was dispersed via the mixing capability of the extruder. The dieand cutting device described in Example 1 was used to produce pellets.Those pellets were subsequently fabricated into thin films at levels of0, 1000, 5000, 10000 and 25000 ppm by dilution into ExxonMobil EXACT3139 polyolefin plastomer (density=0.900 g/cm³; melt index=7.5 dg/min)on the same laboratory scale extrusion equipment used in Example 1.Unlike the films produced in Examples 2-5, the films produced by thecomparative method were rough while exhibiting visually evident specksof sodium nitrite particles. These specks were considered undesirable asa consumer of a packaged product may regard them as contaminants likedirt. Although the resultant films contained large particles, red colordevelopment and fixation in accordance with nitrite concentration wasobserved when fresh meat was sufficiently contacted by the films usingthe method described in Example 1. However, the initial colordevelopment was not homogeneous as large sodium nitrite particles tendedto generate blotches of intense color though the intensity of theblotches tended to diminish with time.

Although the present invention has been described with reference topreferred embodiments, those skilled in the art will recognize thatchanges may be made and formed in detail without departing from thespirit and scope of the invention. It is therefore intended that theforegoing detailed description be regarded as illustrative rather thanlimiting, and that it be understood that it is the following claims,including all equivalents, that are intended to define the scope of thisinvention.

1. A process comprising: a) introducing at least one base polymer intoan extruder; b) heating the base polymer to a temperature sufficient toform a polymer melt; c) introducing into the extruder a liquidcomprising an additive; d) mixing the liquid with the polymer melt inthe extruder to form a blend; e) vaporizing at least a portion of theliquid and removing it from the polymer melt; and f) extruding theadditive containing polymer through an extruder die.
 2. The process ofclaim 1, wherein the base polymer is selected from the group consistingof polyolefin, polyethylene, polypropylene, ethylene vinyl acetate, EMA,EEA, EMAA, polybutene-1, and mixtures thereof.
 3. The process of claim1, wherein the additive is an NO donating compound.
 4. The process ofclaim 1, wherein the additive is a salt.
 5. The process of claim 1,wherein the additive is an inorganic salt.
 6. The process of claim 1,wherein the additive in an undissolved state is a solid at 23° C.
 7. Theprocess of claim 4, wherein the salt is selected from the groupconsisting of nitrite salts, nitrate salts, and mixtures thereof.
 8. Theprocess of claim 4, wherein the salt is selected from the groupconsisting of sodium nitrite, sodium nitrate, and mixtures thereof. 9.The process of claim 1, wherein the polymer is a polyester, ionomer, ornylon.
 10. The process of claim 1, wherein the polymer is heated to atemperature above a melting point of the polymer.
 11. The process ofclaim 1, wherein the polymer is an amorphous polymer and is heated to atemperature above a glass transition point of the polymer.
 12. Theprocess of claim 1, wherein the polymer is heated to a temperature aboveabout 275° F.
 13. The process of claim 1, wherein the polymer is heatedto a temperature less than 100° F. above a melting point of the polymer.14. The process of claim 1, wherein the liquid is water.
 15. The processof claim 1, wherein the liquid comprises between about 20 wt % and about60 wt % additive.
 16. The process of claim 4, wherein the introducedliquid comprises between about 20 wt % and about 42 wt % salt.
 17. Theprocess of claim 4, wherein the introduced liquid comprises betweenabout 30 wt % and about 40 wt % salt.
 18. The process of claim 4,wherein a sufficient amount of the liquid is introduced into the polymermelt to obtain a blend comprising between about 2 wt % and about 10 wt %salt.
 19. The process of claim 4, wherein a sufficient amount of theliquid is introduced into the polymer melt to obtain a blend comprisingbetween about 4 wt % and about 6 wt % salt.
 20. The process of claim 1,wherein at least 10 wt. % of the liquid is removed.
 21. The process ofclaim 1, wherein at least 50 wt. % of the liquid is removed.
 22. Theprocess of claim 1, wherein at least 90 wt. % of the liquid is removed.23. The process of claim 1, wherein at least 98 wt. % of the liquid isremoved.
 24. The process of claim 1, wherein the liquid is removed byventing.
 25. The process of claim 1, wherein the liquid is removed byventing with a vacuum assist.
 26. The process of claim 1 wherein theextruder includes a base polymer feed port and an injection port locatedat a position downstream of the base polymer feed port, wherein thesolution is injected through the injection port.
 27. The process ofclaim 26 wherein the extruder includes a vent located at a positiondownstream from the injection port, further comprising venting vaporthrough the vent.
 28. The process of claim 27 further comprisingproviding at least restriction device downstream of the base polymerfeed port and upstream of the solution feed port.
 29. The process ofclaim 1 further comprising dissolving the additive in water to form theliquid.
 30. The process of claim 1 further comprising dispersing theadditive in water to form a solid in liquid dispersion or emulsion. 31.A pellet formed from the process of claim
 1. 32. A thermoplasticflexible film formed from a thermoplastic polymer made by the process ofclaim
 1. 33. A method of producing packaging for meat comprising:introducing a first polymer into an extruder; introducing a masterbatchof an additive containing polymer into the extruder to form a polymerblend; wherein the masterbatch is made by a) heating a base polymer to atemperature above a melting point of the base polymer to form a polymermelt; b) introducing into the polymer melt a solution comprising a saltand water; c) mixing the solution with the polymer melt in the extruderto form a blend; d) allowing at least a portion of the water to vaporizeand to vent from the extruder; and e) extruding the blend to form themasterbatch; and forming a packaging film from the masterbatchcontaining blend; and fabricating the film into a package, wherein thefilm forms an inner layer of the package.
 34. The process of claim 33,wherein the base polymer is selected from the group consisting ofpolyolefin, polyethylene, polypropylene, ethylene vinyl acetate, EEA,EMA, EMAA, polybutene-1, and mixtures thereof.
 35. The process of claim33, wherein the salt is selected from the group consisting of nitritesalts, nitrate salts, and mixtures thereof.
 36. The process of claim 32,wherein the salt is selected from the group consisting of sodiumnitrite, sodium nitrate, and mixtures thereof.
 37. The process of claim33, wherein a sufficient amount of the solution is introduced into thepolymer melt to obtain a film comprising between about 0.05 wt % andabout 5 wt % salt.
 38. The process of claim 33, wherein a sufficientamount of the solution is introduced into the polymer melt to obtain afilm comprising between about 0.1 wt % and about 2.5 wt % salt
 39. Apackage formed from the process of claim 33.